Method, device and software application for transmitting data packets in a communication system

ABSTRACT

The invention relates to a method for transmitting a plurality of data packets to a receiver in a data communication system. The method comprises the steps of transmitting one or more data packets from a list of data packets to be transmitted ( 300 ); determining whether an acknowledgment is received for each transmitted data packet ( 301 ), and further comprises the following steps executed when it is determined at the determining step that an acknowledgement has not been received for at least one data packet, referred to as an unacknowledged data packet: selecting one or more additional data packets from the list of data packets to be transmitted ( 302 ); generating one or more parity packets by encoding a block of data containing a combination of the selected one or more additional data packets and at least one unacknowledged data packet using a forward error correction scheme ( 304 ); and transmitting at least one of the generated parity packets ( 305 ). 
     The invention also relates to software applications for transmitting a plurality of data packets and for receiving them. Furthermore, the invention relates to a transmitting device and a receiving device implementing respectively the software application for transmitting the plurality of data packets and the software application for receiving those data packets, and to a memory medium for storing the code of such software applications.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to data communication systems. The presentinvention is especially applicable for transmitting data packets in suchcommunication systems.

2. Description of the Background Art

Error control techniques are extensively used in data communicationsystems to restore data integrity when data is corrupted by transmissionerrors or is lost. Automatic-Repeat-Request (ARQ) schemes areretransmission-based error control mechanisms commonly used to deliverhighly reliable data to destination.

In ARQ, an error-detecting code, a checksum for example, is used incombination with a certain retransmission protocol. Corrupted or lostdata packets are transmitted repeatedly until correctly received by thereceiver. Feedback information (acknowledgments), sequence numbering andtimers are used to ensure correct delivery of data packets at thereceiver.

TCP (Transport Control Protocol) is an example of a widely used protocolimplementing an ARQ error-control mechanism. In this protocol, thesending device uses acknowledgments (ACKs) it receives to determinewhich data packets (segments) have reached the receiver, and providesreliability by retransmitting lost packets. The sending deviceidentifies the loss of a packet either by the arrival of severalacknowledgments acknowledging a same data packet (duplicate ACKs) or theabsence of an acknowledgment for the packet before a timeout. At thereceiving device, the sequence numbers are used to correctly orderpackets that may be received out of order and to eliminate duplicates.Damage is handled by adding parity data using an error detecting code(checksum) to each packet transmitted, checking it at the receiver, anddiscarding damaged packets.

One common drawback of retransmission-based error control mechanisms,including TCP one, is that the acknowledgment packets also suffer fromthe impairments of the communication channel. The sending device has todecide if an already transmitted data packet shall be retransmitted ornot based on the available feedback information, and the unreliabilityof that feedback information may cause unnecessary retransmission ofdata packets and creation of duplicates at the receiver. In addition toa waste of channel bandwidth, duplication of data packets may furtherdegrades transmission conditions in the communication system byaggravating congestions for example.

There exist various conditions under which even a well designed ARQprotocol leads to the reception of multiple copies of a data packet atthe receiver.

For example, it may happen that an acknowledgment packet is lost. Inthis case, even if the data packet is correctly delivered to thereceiving device, the sending device is not aware of such delivery andthus retransmits the data packet after timeout. If the retransmission issuccessful, the data packet is delivered twice to the receiver.

Another example relates to the use of independent error controlmechanisms at different layers. Consider an end-to-end reliabletransport protocol, such as TCP, which is tuned to perform well intraditional networks where packet losses occur mostly because ofcongestion. When some links are wireless or use any other lossy channel,they suffer from significant losses due to bit errors. Because of thesignificant time required to recover from losses at the link layer(generally, wireless network standards accept up to 16 retransmissions),acknowledgment or data packets belonging to the end-to-end protocolmight be delayed more than usual. Even if the data packets finally reachthe receiver or the corresponding acknowledgement packets reach thesending device, it's likely that the data packets are assumed to be lostby the sending device and retransmitted.

Yet another example relates to variable transmission delay thatdifferent packets may experience in a multi-path communication systemwhich causes out of sequence reception of packets. Different packets maybe transmitted over different paths. Each path may contain a differentnumber of intermediate devices (hubs, switches, routers, and so on) andeach device may be working at a different processing speed. Packetsfollowing congested paths or transferred via intermediate devices havinglow processing speed could experience long delays and thus can beconsidered as lost by end devices. TCP, for example, maintains a runningaverage of the estimated roundtrip delay and the mean linear deviationfrom it. The sending device identifies the loss of a packet either bythe arrival of several duplicate acknowledgments (this may happen if apacket is delayed for too long) or the absence of an acknowledgment forthe packet within a predetermined period of time equal to the sum of thesmoothed round-trip delay and four times its mean deviation.

SUMMARY OF THE INVENTION

The present invention has been made for enhancing the performance of thecommunication system by avoiding useless retransmissions. Particularly,the present invention intends to solve the problem of receiving multiplecopies of a packet at the receiver end.

According to a first aspect of the present invention, there is provideda method for transmitting a plurality of data packets to a receiver in acommunication system comprising the steps of transmitting by atransmitter one or more data packets from a list of data packets to betransmitted and determining whether an acknowledgment is received foreach transmitted data packet. When it is determined at the determiningstep that an acknowledgement has not been received for at least one datapacket, referred to as an unacknowledged data packet, the followingsteps are executed: selecting one or more additional data packets fromthe list of data packets to be transmitted; generating one or moreparity packets by encoding a block of data containing a combination ofthe selected one or more additional data packets and at least oneunacknowledged data packet using a forward error correction scheme; andtransmitting at least one of the generated parity packets.

An advantage of this method is that if the unacknowledged data packetswere actually correctly received by the receiver, they will be used incombination with the parity packets to recover the additional one ormore data packets that were not, and will never be, transmitted. Noduplicated packets are thus received by the receiver and all transmittedinformation is exploited to generate the additional data packets.

Advantageously, the transmitting method further comprises the steps ofdetermining whether an acknowledgment is received for each data packetcontained in the block of data, and transmitting at least one of theselected data packets if at least one data packet contained in saidblock of data is unacknowledged.

Parity packets are thus used by the receiver to recover theunacknowledged data packets, if they were indeed not correctly received,using the selected data packets transmitted later on by the transmitter.Again, no duplicated packets are received by the receiver and alltransmitted information is exploited to recover the unacknowledged datapackets.

According to a preferred embodiment of the invention, the forward errorcorrection scheme uses a maximum distance separable (MDS) code. Withsuch a code, one parity packet contains enough information to recoverone data packet. It is thus easier to determine how many parity packetsneed to be transmitted by the transmitter considering the number ofunacknowledged data packets in order for the receiver to have just thenecessary and sufficient information to recover all the data packets.Furthermore, MDS codes are efficient codes.

In a particular mode of implementation, the number of parity packetstransmitted at the transmitting step is at least equal to the number ofunacknowledged data packets used to generate said parity packets. Thus,the receiver is able to recover all the unacknowledged data packets ifthey have actually not been received.

According to a preferred mode of implementation, the number of paritypackets transmitted at the transmitting step, the number ofunacknowledged data packets used to generate said parity packets and thenumber of selected data packets are all equal which makes the managementof the transmission of parity and data packets simple.

According to a preferred embodiment of the invention, one parity packetis generated by computing a bitwise XOR of one selected data packet andone unacknowledged data packet. Very simple coding can thus be obtainedof the data packets.

According to another embodiment of the invention, the parity packets aregenerated by applying a Reed-Solomon code over a Galois field of size2^(m), m being the number of binary elements of the symbols of the code,wherein said code is used for encoding each word j formed by theconcatenation of the jth symbol of each data packet. Reed-Solomon codesare well known MDS codes that provide efficient encoding.

According to a second aspect of the present invention, there is provideda method for receiving a plurality of data packets from a transmitter ina communication system comprising the steps of: receiving one or moreparity packets from the transmitter, said parity packets having beengenerated by encoding, using a forward error correction scheme, a blockof data containing a combination of a first set of data packetstransmitted by the transmitter and a second set of data packets to betransmitted by said transmitter; partially reconstructing an encodedblock of data associated to the one or more received parity packetsbased on received data packets belonging to the first and second sets ofdata packets; determining whether the block of data can be recovered bydecoding the encoded block of data reconstructed at the reconstructingstep; and if the block of data can be recovered, sending to thetransmitter an acknowledgement corresponding to each data packetincluded in said recovered block of data.

The invention also relates to software applications for transmitting aplurality of data packets and for receiving them.

Furthermore, the invention relates to a transmitting device and areceiving device implementing respectively the software application fortransmitting the plurality of data packets and the software applicationfor receiving those data packets, and to a memory medium for storing thecode of such software applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a communication system where the presentinvention can be implemented;

FIG. 2 is a block diagram illustrating a schematic configuration of acommunication apparatus representing a transmitting device or areceiving device adapted to incorporate the invention;

FIG. 3 is a flowchart illustrating a transmitting method of data packetsaccording to the present invention as to be implemented at the sendingdevice;

FIG. 4 depicts the format of an encoded block of data;

FIG. 5 depicts an example of an encoding scheme of data packets;

FIGS. 6 a, 6 b and 6 c depict examples of control information that canbe transported in the header of a packet; and

FIG. 7 is a flowchart illustrating a receiving method of packetsaccording to the present invention as to be implemented at the receivingdevice.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, a detailed description will be given of embodiments ofthe present invention with reference to the accompanying drawings.

FIG. 1 is an example of a communication system where the presentinvention can be implemented. A sending device (101) transmits aplurality of data packets to a receiving device (102) through atransmission channel (100) that may drop or corrupt the data packets.Feedback information is transmitted from the receiving device to thesending device in order to provide status information on the receiveddata packets. The present invention according to its first aspect isimplemented at the sending device for transmitting the plurality of datapackets and according to its second aspect is implemented at thereceiving device for receiving the plurality of data packets.

FIG. 2 is a block diagram illustrating a schematic configuration of acommunication apparatus representing a transmitting device or areceiving device adapted to incorporate the invention. Reference numeral202 is a RAM which functions as a main memory, a work area, etc., of CPU201, and the memory capacity thereof can be expanded by an optional RAMconnected to an expansion port (not illustrated). CPU 201 is capable ofexecuting instructions on powering up of the communicating apparatusfrom program ROM 203. After the powering up, CPU 201 is capable ofexecuting instructions from main memory 202 relating to a softwareapplication after those instructions have been loaded from the programROM 203 or the hard-disc (HD) 206 for example. Such softwareapplication, when executed by the CPU 201, causes the steps of theflowcharts shown in FIG. 3 and/or 7 to be performed.

Reference numeral 204 is a network interface that allows the connectionof the communication apparatus to the communication channel. Datapackets are written to the network interface for transmission or readfrom the network interface for reception under the control of thesoftware application running in the CPU 201. Reference numeral 205represents a user interface to display information to, and/or receiveinputs from, a user.

The transmitting method of packets according to the present invention asto be implemented at the sending device is illustrated by the flowchartof FIG. 3. This method allows the transmission of packets from a list Lof packets to be transmitted.

The list L represents for example a buffer memory containing packets ofdata. A packet is written in the buffer if it is to be transmitted, andis removed from the buffer after being transmitted. Consequently, thebuffer memory, and thus the list L, is assumed to be continuouslyupdated by the sending device in order to contain only packets that arewaiting for transmission. The way of implementing the list L describedabove is not limitative and any other implementation can be used, suchas using one buffer memory for storing all packets and tagging thosethat are waiting for transmission.

The sending device transmits data packets from the list L (step 300) tothe receiving device and monitors the received acknowledgments todetermine if there exists any unacknowledged data packets (step 301). Itshould be noted that the conditions under which a packet is consideredas unacknowledged remain the same as those known to skilled persons inthe art when implementing retransmission based protocols. Thoseconditions include, obviously, the absence of acknowledgement within apredetermined period of time, but could be also any otherprotocol-dependent conditions such as receiving multipleacknowledgements for a given packet or any specific feedback signallingfrom the receiving device.

For example, the determining step 301 may comprise a step of measuringelapsed time since transmitting a data packet, said data packet beingconsidered as unacknowledged if the elapsed time is greater than apredetermined value. In an alternate embodiment, the determining stepmay comprise a step of counting the number of acknowledgements receivedfor data packets transmitted after the transmission of a given datapacket, said given data packet being considered as unacknowledged if thecounted number is greater than a predetermined value.

If it is determined at the determining step 301 that all transmitteddata packets have been acknowledged, the sending device continuestransmitting data packets from the list L, if packets waiting fortransmission remain. However, if it is determined that at least one datapacket is unacknowledged, a recovery procedure is executed according tothe present invention following the steps 302 to 307.

The pace at which the steps 300 and 301 are executed depends on thedesign choice of the communication protocol (window size, etc.). It ispossible for the sending device to continuously monitor any receivedacknowledgment.

At step 302, one or more data packets are selected from the list L ofpackets to be transmitted. Let k1 be the number of those selected datapackets, where k1 is an integer greater than or equal to 1. At step 303,one or more packets are selected among the unacknowledged packets. Letk2 be the number of those selected unacknowledged packets, where k2 isan integer greater than or equal to 1.

The step 304 of generating parity packets is as follows. The k1 datapackets to be transmitted and the k2 unacknowledged packets are combinedto form a block of data of size k=k1+k2 packets. The block of data isafterwards encoded using a forward error correcting (FEC) code to forman encoded block of data of size n=k+h packets, where h is the number ofparity packets generated by the FEC code. The parity packets, whencorrectly received by the receiving device, can be used to recover oneor more missing data packets if is it within the correcting capacity ofthe code.

Taking into account the capacity of the code, a number h1 (h1<h) ofparity packets is transmitted at step 305 to the receiving device, h1being such that it is just enough to recover the k1 packets consideringthat all k2 unacknowledged packets have actually been correctly receivedby the receiving device. Consequently, duplicated packets are avoided atthe receiving device. If however it happens that part or all of the k2unacknowledged packets were actually not received by the receivingdevice, the sending device will transmit the required number of datapackets chosen among the k1 packets to be transmitted for the receivingdevice to be able to recover the initially not received part or all ofthe k2 packets.

The number of generated parity packets (h) and the number of transmittedones (h1) are preferably equal, but it is not a requirement. A givenforward error correcting scheme may generate more parity packets thannecessary and part of them may be enough to implement the transmittingand receiving methods according to the present invention.

The sending device determines at the step 306 if all the k1 and k2packets forming the block of data have been acknowledged. The sendingdevice has to wait preferably until a timeout associated to the lasttransmitted parity packet is detected, to reduce the probability ofmissing an acknowledgment.

If all the data packets (k) of the data block have been acknowledged,the recovery of the k1 data packets was successful, and then those datapackets are removed from the list L of data packets to be transmitted(step 308). Afterwards, the step 300 of transmitting data packets fromthe updated list L is executed.

If not all the data packets (k) of the data block have beenacknowledged, the sending device transmits at least one of the selectedk1 data packets at a step 307.

More details on the encoding scheme of the data block are providedhereinafter.

FIG. 4 depicts the format of an encoded block of data (400) encoded byusing a systematic block code. Reference 401 indicates the k=k1+k2 datapackets (X_(i)) forming the data block, and reference 402 refers to theh parity packets (P_(i)) generated by the encoding of the data block.All packets are assumed to have a fixed size of S symbols (x_(i,j)) asindicated by reference 403. This is not however a requirement, sincepadding data can be used to obtain fixed size packets.

Typically, a block code C takes a K-symbols information word andtransforms this into an N-symbols codeword, where N>K and N-Krepresenting the amount of redundancy added by the code. The symbols areelements of an “alphabet” of finite size. The most common symbols arebinary elements belonging the alphabet {0, 1} of size 2, and bytes (8bits) belonging to the alphabet {0, . . . , 255} of size 2⁸=256.

The correcting capacity of a code is defined by its minimum distance Dand depends on the rate of the code (K/N) and how the parity symbols aregenerated from the information symbols. The greater the minimumdistance, the higher is the correcting capacity of the code. The highestcorrecting capacity is obtained when D=N−K+1. Codes providing suchminimum distance are called maximum distance separable (MDS) codes. AReed-Solomon code is an example of an MDS code.

Different techniques can be used to generate parity packets from aplurality of data packets using a code C(N, K).

A first technique is to encode the data packets at a symbol level bychoosing the parameters K=k*S and N=n*S of the code C(N, K). Thistechnique requires the use of a long length code if the number k ofpackets to encode is high. For example, if the size of a packet is 512bytes (a byte representing a symbol), a code C(3072, 2048) would beneeded in order to generate two parity packets from four data packets(k=4).

A second technique consists of increasing the symbol size (number ofbinary elements) to reduce the number of symbols per packet, the upperlimit being equal to the size of one packet. This technique may lead toa tremendously large alphabet making the encoding and decoding processescomplex (a symbol of size 2 bytes corresponds to an alphabet of size2¹⁶=65536).

In a preferred embodiment, a third technique is used for the encoding ofdata packets. The encoding scheme principle based on this technique isdepicted in FIG. 5.

The k data packets (X_(i)) are organized into a matrix where each columncorresponds to a data packet (500) and where each line (501) is formedby a plurality of symbols (x_(i,j)), each of them taken at a givenposition (j) from every data packet. Each line of k symbols is encodedwith a code C(n, k) to generate n−k=h parity symbols (502). The paritysymbols are also organized into a matrix where each column (503)represents a parity packet. This technique allows the use ofconventional codes to encode a plurality of data packets of arbitrarysize by applying the code C(N=n, K=k) S times, where the number (k) ofdata packets per data block equates the number of symbols perinformation word to be encoded.

The encoder has to inform the decoder about the coding parameters. Theparameters N and K of the code can be set once for all and shared duringa setting step between the encoder at the sending device and the decoderat the receiving device. The number of data packets in the data block(k) can also be set to a fixed size or kept dynamic from one data packetto another. It should be noted that even if a data block of shortersize, i.e. k0<k, is used at a given time, it's still possible to use thesame encoding and decoding hardware C(N, K) as the one for a data blockof size k by using code shortening techniques known to persons skilledin that art.

Protocol related control information need also be exchanged between thesending device and the receiving device. Such control information allowsthe sending device to tell the receiving device exactly which datapackets have been used to generate which parity packets, and makes thereceiving device distinguish between packets belonging to differentencoded data blocks when transmitted consecutively.

A packet, either data or parity, contains a payload field and a headerfield. The payload field contains information meaningful for higherapplications, the header field contains mainly control information.

FIGS. 6 a, 6 b and 6 c depict examples of control information that canbe transported in the header of the packets for supporting thetransmitting and receiving methods according to the present invention.

All data packets are numbered by using a sequence number (SN) asrequired by some communication protocols, e.g. TCP, before beingtransmitted. The sequence number is included in a field of predeterminedsize located in the header of every packet. Numerals 601 and 605represent sequence numbers of the stream of data packets 602.

Parity packets can either be sent in the same stream as the data packetsor in a separate stream. When sent as a separate stream, the paritypackets have their own sequence number space as shown by referencenumerals 603. A payload type field could be added in the packets headerto differentiate between parity and data.

It should be noted that according to the present invention data packetsbelonging to a given stream are not all systematically encoded togenerate parity packets. Indeed, if there are no unacknowledged datapackets there is no need to generate parity packets, although it may bedone preventatively. Data block construction depends on the detection ofunacknowledged data packets. Consequently, data packets belonging to adata block do not necessarily possess consecutive sequence numbers asindicated by reference 604. In FIG. 6 a, data packets 11 and 14 aresupposed to be unacknowledged, and data packets 17 and 18 are to betransmitted data packets.

All data packets forming a given data block are taken from a range M ofdata packets of the main stream 602. M can be set to a predeterminedvalue, the parameters k1 and k2 of the code being then adapted to thenumber of unacknowledged data packets detected within that range, orvariable to reach predetermined values for the parameters k1 and k2 ofthe code.

Setting the size of the data block is a tradeoff between overhead,delay, and recoverability. If unacknowledged data packets are notfrequent, it might be enough to choose a code C(3, 2) which consists ofadding one packet to be transmitted to one unacknowledged data packet togenerate one parity packet. This operation is repeated for everyunacknowledged data packet detected.

A C(3, 2) code can be implemented very simply and efficiently by use ofan exclusive-OR operation (XOR). One parity packet is generated bycomputing a bitwise XOR of one selected data packet and oneunacknowledged data packet. The recovery (decoding) of one data packetcan be simply performed by repeating the bitwise XOR operation betweenthe parity packet and the other one of the data packets.

FIGS. 6 b and 6 c represent two alternative solutions for numbering thepackets of a data block (in addition to the sequence number SN). Othersolutions may also exist.

In FIG. 6 b, reference numeral 606 represents a block sequence number(BSN). This sequence number allows the receiving device to distinguishbetween data packets included in a data block and those which are not,and to not group together data packets belonging to different datablocks. For example, the block sequence number could use 4 bits fornumbering from 1 to 15; a specific BSN value (0) could be assigned todata packets not participating to the generation of parity packets.Reference numeral 607 represents a local sequence number (LSN). Thissequence number allows the receiving device to position appropriatelythe corresponding packet within the encoded data block for decoding.

FIG. 6 c depicts an alternative solution to associate the parity packetswith the data packets used to generate those parity packets by using abitmask containing M bits (608). If bit i in the bitmask is set to 1,the packet with sequence number SNB+i was used to generate this paritypacket. SNB is called the sequence number base, and is sent in theparity packet as well. The bitmask and payload type (parity/data) aresufficient to signal arbitrary parity based forward error correctionschemes with little overhead.

The receiving method of packets according to the present invention as tobe implemented at the receiving device is illustrated by the flowchartof FIG. 7. This method allows the recovery of missing data packets froma list L of data packets transmitted by the sending device, whensufficient number of parity packets have been received, by avoiding thereception of duplicated data packets.

At step 700, parity and data packets are received by the receivingdevice. The receiving device uses control information included in theheader of the received packets to identify the existence of a block ofdata (cf. FIG. 6). The block of data is reconstructed based on thatcontrol information (701). When packets are detected as missing, theircorresponding symbols are marked as erased. The encoded block of data isalways partially reconstructed, either because of not transmitted orlost data packets or parity packets, since the aim of the sending deviceis to transmit just enough information to reach the recoverabilitythreshold for the data packets of the data block.

Considering the missing packets (and thus erased symbols), the sendingdevice determines if enough information is available to recover allmissing data packets (702). For example, if a Reed-Solomon code is usedto encode k data packets and produces n−k parity packets, all missingpackets can be recovered if their number is not greater than n−k (numberof parity packets). When using an exclusive or operation, only onepacket can be recovered.

Feedback information from the receiving device informs the sendingdevice about the recoverability status of the data block, and thustriggers the transmission by the sending device of further packets ifnecessary.

If it is determined at step 702 that the data block can be recovered,the recovery is effected at step 703 by decoding the encoded data blockusing the parity packets. Data packets contained in the data block arethen retrieved at step 704 and corresponding acknowledgments are sent tothe sending device at step 705.

Advantageously, an acknowledgement sent at the sending step 705 foracknowledging a given data packet contains a flag indicative whethersaid given data packet has been recovered following the decoding of theencoded block of data.

If it is determined at step 702 that the data block cannot be recovered,the receiving device waits for additional packets belonging to the sameencoded data block. When such packets are received (706), they are addedto the encoded data block by filling missing positions (707) and thenthe step 702 is executed again to determine if the recovery is nowpossible.

Correctly received data packets are always acknowledged by the receivingdevice. Different strategies can be used however regarding theacknowledgment of parity packets. Preferably, parity packets areacknowledged by the receiving device to make the sending device aware ofhow far the receiving is device from the recoverability threshold. Thesending device will then transmit only the sufficient number of packetsin order for the receiving device to be able to recover all the missingdata packets by taking into account the parity packets correctlyreceived.

1. A method for transmitting a plurality of data packets to a receiverin a communication system comprising the steps of: transmitting one ormore data packets from a list of data packets to be transmitted (300);and determining whether an acknowledgment is received for eachtransmitted data packet (301); the method being characterized in that itfurther comprises the following steps executed when it is determined atthe determining step that an acknowledgement has not been received forat least one data packet, referred to as an unacknowledged data packet:selecting one or more additional data packets from the list of datapackets to be transmitted (302); generating one or more parity packetsby encoding a block of data containing a combination of the selected oneor more additional data packets and at least one unacknowledged datapacket using a forward error correction scheme (304); and transmittingat least one of the generated parity packets (305).
 2. Method accordingto claim 1 characterized in that it further comprises the steps of:determining whether an acknowledgment is received for each data packetcontained in the block of data (306); and transmitting at least one ofthe selected data packets (307) if at least one data packet contained insaid block of data is unacknowledged.
 3. Method according to claim 1characterized in that the determining step comprises a step of measuringelapsed time since transmitting a data packet, said data packet beingconsidered as unacknowledged if the elapsed time is greater than apredetermined value.
 4. Method according to claim 1 characterized inthat the determining step comprises a step of counting the number ofacknowledgements received for data packets transmitted after thetransmission of a given data packet, said given data packet beingconsidered as unacknowledged if the counted number is greater than apredetermined value.
 5. Method according to claim 1 characterized inthat the forward error correction scheme uses a maximum distanceseparable code.
 6. Method according to claim 5 characterized in that thenumber of parity packets (h1) transmitted at the transmitting step is atleast equal to the number of unacknowledged data packets (k2) used togenerate said parity packets.
 7. Method according to claim 6characterized in that the number of parity packets (h1) transmitted atthe transmitting step, the number of unacknowledged data packets (k2)used to generate said parity packets and the number of selected datapackets (k1) are all equal.
 8. Method according to claim 7 characterizedin that k1, k2 and h1 are all equal to one, and that one parity packetis generated by computing a bitwise XOR of one selected data packet andone unacknowledged data packet.
 9. Method according to one of claims 5to 7, characterized in that the maximum distance separable code is aReed-Solomon code having the parameters(n=k1+k2+h, k=k1+k2) over aGalois field of size 2^(m), m being the size of the symbols of the codeand h an integer greater than or equal to h1, wherein said code is usedfor encoding each word j (501) formed by the concatenation of the jthsymbol of each data packet (500).
 10. A method for receiving a pluralityof data packets from a transmitter in a communication system comprisingthe steps of: receiving one or more parity packets from the transmitter(700), said parity packets having been generated by encoding, using aforward error correction scheme, a block of data containing acombination of a first set of data packets transmitted by thetransmitter and a second set of data packets to be transmitted by saidtransmitter; partially reconstructing an encoded block of dataassociated to the one or more received parity packets based on receiveddata packets belonging to the first and second sets of data packets(701); determining whether the block of data can be recovered bydecoding the encoded block of data reconstructed at the reconstructingstep (702); and if the block of data can be recovered, sending to thetransmitter an acknowledgement corresponding to each data packetincluded in said recovered block of data (705).
 11. Method according toclaim 10, characterized in that, if the block of data cannot berecovered, the following steps are executed: receiving one or moreadditional packets belonging to the encoded block of data (706); andfully reconstructing the encoded block of data by adding the receivedone or more additional packets to the encoded block of data (707). 12.Method according to claim 10 or 11, characterized in that it furthercomprises the steps of: decoding the encoded block of data to recoverthe block of data (703); retrieving data packets contained in therecovered block of data (704).
 13. Method according to claim 10,characterized in that an acknowledgement sent at the sending step foracknowledging a given data packet contains a flag indicative whethersaid given data packet has been recovered following the decoding of theencoded block of data.
 14. A software application for transmitting aplurality of data packets to a receiver in a communication system,arranged such that when executed the following steps are performed:transmitting one or more data packets from a list of data packets to betransmitted (300); and determining whether an acknowledgment is receivedfor each transmitted data packet (301); the software application beingcharacterized in that the following further steps are performed when itis determined at the determining step that an acknowledgement has notbeen received for at least one data packet, referred to as anunacknowledged data packet: selecting one or more additional datapackets from the list of data packets to be transmitted (302);generating one or more parity packets by encoding a block of datacontaining a combination of the selected one or more additional datapackets and at least one unacknowledged data packet using a forwarderror correction scheme (304); and transmitting at least one of thegenerated parity packets (305).
 15. A memory medium storing the code ofthe software application according to claim
 14. 16. A softwareapplication for receiving a plurality of data packets from a transmitterin a communication system, said software application being arranged suchthat when executed it performs the steps of: receiving one or moreparity packets from the transmitter (700), said parity packets havingbeen generated by encoding, using a forward error correction scheme, ablock of data containing a combination of a first set of data packetstransmitted by the transmitter and a second set of data packets to betransmitted by said transmitter; partially reconstructing an encodedblock of data associated to the one or more received parity packetsbased on received data packets belonging to the first and second sets ofdata packets (701); determining whether the block of data can berecovered by decoding the encoded block of data reconstructed at thereconstructing step (702); and if the block of data can be recovered,sending to the transmitter an acknowledgement corresponding to each datapacket included in said recovered block of data (705).
 17. A memorymedium storing the code of the software application according to claim16.
 18. A transmitting device for transmitting a plurality of datapackets in a communication system, comprising: a transmission unit (204)for transmitting one or more data packets from a list of data packets tobe transmitted; and a determination unit (201) for determining whetheran acknowledgment is received for each transmitted data packet; thedevice being characterized in that it further comprises the followingunits implemented when it is determined by the determination unit thatan acknowledgement has not been received for at least one data packet,referred to as an unacknowledged data packet: a selection unit (201) forselecting one or more additional data packets from the list of datapackets to be transmitted; a coding unit (201) for generating one ormore parity packets by encoding a block of data containing a combinationof the selected one or more additional data packets and at least oneunacknowledged data packet using a forward error correction scheme; anda transmission unit (204) for transmitting at least one of the generatedparity packets.
 19. A receiving device for receiving a plurality of datapackets from a transmitter in a communication system, comprising: areceiving unit (204) for receiving one or more parity packets from thetransmitter, said parity packets having been generated by encoding,using a forward error correction scheme, a block of data containing acombination of a first set of data packets transmitted by thetransmitter and a second set of data packets to be transmitted by saidtransmitter; a reconstruction unit (201) for partially reconstructing anencoded block of data associated to the one or more received paritypackets based on received data packets belonging to the first and secondsets of data packets; a determination unit (201) for determining whetherthe block of data can be recovered by decoding the encoded block of datareconstructed at the reconstructing step; and a transmitting unit (204)for sending to the transmitter, if the block of data can be recovered,an acknowledgement corresponding to each data packet included in saidrecovered block of data.